In recent years, organic EL display panels, which offer sharp contrasts and wide angles of vision and emit light on their own, so do not need backlights and are therefore suitable for reduction of thickness, are attracting attention.
Organic EL display panels are now entering the commercial stage in inch sizes. Advances in materials, production technology, and drive circuits have led to a succession of releases of prototype panels of the 13 to 17 inch sizes in recent years.
Organic EL elements have curved current-voltage characteristics, like diodes. The luminance-current characteristics have linear proportional relationships.
In this way, organic EL elements and thin film transistors (TFT) have threshold voltages and have large variations. For this reason, in organic EL display panels, it is proposed to use current controlled drive circuits, having proportional relationships with the luminance, to reduce uneven luminance of the display panels.
In liquid crystal panels for personal computers, televisions, and other applications, multi-bit high gradation display is required.
With just low temperature polysilicon TFT circuits formed on the panel, fabrication of multi-bit digital/analog converters (DAC) and other complex circuits is difficult, so the practice has been to bond voltage output type driver IC's, for driving vertical direction data lines, to the peripheral portions of the panel to form a module.
In the drive circuit of a large size display panel, the practice has been to use a plurality of drivers to drive the screen divided. In such a case, if the characteristics vary among drivers, there is the problem that steps of the luminance are generated at the border lines of the screen driven by division.
In the case of a liquid crystal display, the data line driver is a voltage output type. For this reason, it is possible to make the luminance step very small by the simple method of commonly connecting an interconnect line of the reference voltage between driver integrated circuits (driver IC's).
FIG. 1 is a circuit diagram of a reference voltage generation circuit used in a data line driver etc. of a liquid crystal display.
This reference voltage generation circuit generates nine reference voltages of V0, V8, . . . , and V64 by the resistance division of resistor elements R0 to R7 connected in series between a supply line of a power supply voltage VDD and a ground line GND. Then, by further fine interpolation among these reference voltages by DAC etc., for example, by equally dividing it by 8, voltage outputs of 64 scales can be obtained.
When providing this reference voltage generation circuit in the driver IC, even if the absolute value of the resistance varies for every driver IC, the reference voltage output is determined by the resistance ratio, so there is almost no variation among driver IC's.
FIG. 2 is a view for explaining an inter-driver IC's connection system of the reference voltage in a voltage output type data line driver.
In this case, a display panel PNL is driven by dividing it by n number of anode drivers IC's 1 to n.
Even if there is variation in the reference voltage outputs among driver IC's, as shown in FIG. 2, when the terminals of the reference voltages of all driver IC's are connected for each of the reference voltages V0, V8, . . . , and V64, the voltage averaged for each reference voltage will be supplied to all driver IC's 1 to n.
For this reason, a luminance step of a level causing a problem will not be generated at border lines of a screen driven by division.
In the case of an organic EL display, a current output type is suitable as a data line driver.
In a current output type driver IC, suitable for an organic EL display, if supplying a common reference voltage to the driver IC's and then having each driver IC perform voltage-current conversion to generate the reference current as described above, the reference current will vary among the driver IC's due to the variation of the offset voltage of the operational amplifiers and resistor elements configuring the voltage-current conversion circuits. Further, even if performing the voltage-current conversion before the final output, the output current will vary among output terminals.
In order to reduce the factors behind this current variation, an organic EL full color module drive system, employing the current connection system in a current output type anode driver IC, has been proposed (refer to for example Non-patent Document 1: “Development of Organic EL Full Color Module Drive System”, Pioneer R&D, vol. 11, no. 1, page 29-36, 2001, Ochi, Sakamoto, Ishizuka, Tsuchida).
FIG. 3A is a view of this organic EL full color module drive system. In this drive system as well, a display panel OPNL is driven by division by n number of anode driver IC's 11 to 1n. 
In the present drive system, when providing a reference current source at each of the driver IC's to set the current, the reference currents will subtly differ due to the individual differences in the performances of the IC's or the current setup parts, so sometimes luminance steps will be generated in units of IC's. Further, using a variable resistor for each IC to adjust for each IC is unsuitable for mass production; therefore, by using the closest current output of the adjacent IC as the reference current, the variation of the set currents can be absorbed and the luminance steps can be eliminated.
According to this current connection system, a step of adjusting luminance among drivers becomes unnecessary. Also, the number of interconnects of the reference current on the panel can be made relatively small.
As explained above, in the current connection system shown in FIG. 3A, luminance steps corresponding to the border lines of horizontally adjacent drivers can be eliminated.
As shown in FIG. 3B, however, a reference current IREF of the driver on the left end and a reference current IREF(n−1) on the right end become different by addition of the n number of current variations in the driver IC's.
In a large size display device, not only is the display panel driven by dividing it in the lateral direction, but also the data lines on the panel are vertically divided at the ½ positions in the vertical direction to halve the interconnect capacitances of the data lines. Together with this, the drive frequency is reduced by vertically arranging drivers and driving them in parallel and by halving the number of scanning lines which must be driven by each driver.
In such case, with the current connection system, luminance steps are sometimes generated at the vertical borders of the display panel.
As described above, with the conventional method of supply of the reference current, it is difficult to realize a large size, high gradation display type organic EL display.
For this reason, in organic EL display panels, the appearance of current output type data line drivers (source drivers) suitable for driving organic EL elements has been awaited.